1. Field of the Invention
This invention relates generally to aerodynamic surfaces and, more particularly, to improved constructions and control schemes for such aerodynamic surfaces which provide for aerodynamic control and for significant reductions in noise in the case of rotor blades.
2. Description of Related Art
Conventional helicopters in descent flight conditions frequently generate an impulsive noise signature which is commonly referred to as blade-vortex interactions (BVI) noise or "blade slap." BVI noise is generated by blade tip vortices, which interact with the rotor blades. Unfortunately, it is typically within a frequency range which is highly important to human subjective response. Additionally, it is easily detected electronically at large distances, thus increasing the vulnerability of military rotorcraft. Consequently, a reduction in the BVI noise intensity and changes in the noise signature, using active and/or passive noise control techniques, is desirable to the rotorcraft industry, which is challenged by today's stringent military and civilian acoustic regulations.
There are three possible measures which may be taken to reduce BVI noise. Namely, the tip vortex strength may be weakened, the separation distance between the blade and the tip vortex may be increased, and/or the blade geometry may be altered. The result of these measures is a decrease in the strength of the interaction between the rotor blade and the tip vortices. Existing devices which have been used for reducing BVI noise include the use of a blade-mounted trailing edge flap which seeks to change the strength of the tip vortex and hence the intensity of BVI and the use of Higher Harmonic root pitch control (HHC), which seeks to change the blade/vortex distance, and thus the local aerodynamic conditions, through blade pitch changes.
Other control means concentrate primarily on reducing the strength of the tip vortex through blade tip geometric modifications. Typical examples are the use of leading and trailing edge sweep, the use of blade anhedral, spoilers, and the use of a subwing concept. All of these examples, excluding HHC, may be classified as passive control techniques. An example of another active control technique would be the use of tip air mass injection, which again has the purpose of weakening the blade tip vortices. Tip air mass injection involves introducing a high energy air jet at the tip of the blade, aimed at the center or core of the tip vortex with the sole purpose of diffusing or weakening its strength.
Each of the prior art solutions to BVI noise has been at least partially unsuccessful, either because of ineffectiveness or because of the solution's detrimental side effects with respect to the flight characteristics and efficiency of the rotorcraft. For example, HHC methods change the aerodynamic conditions along the entire blade in order to reduce BVI noise, due to the change in blade pitch. Passive BVI noise control methods are not adaptable to changing BVI conditions throughout the flight regime, which are associated with changes in descent rate and forward flight speed. Additionally, most of the passive prior art solutions to the BVI problem are deployed at all times, whether or not needed, often degrading flight performance unnecessarily.
In addition to problems associated with reducing BVI noise generated by rotor blades, a more general problem exists with providing controllable aerodynamic surfaces on rotor blades, wings, engine inlets, and nozzles. Movable control surfaces placed on these aerodynamic surfaces have included flaps, slats, spoilers, ailerons, elevators, and rudders. Although these control surfaces can mechanically alter the geometry of the original aerodynamic device, they are limited in ability to respond quickly and efficiently. Prior art mechanical control surfaces can add mechanical complexity to the aircraft, can compromise structural integrity, can complicate manufacturing, and can compromise radar detectability.
A synthetic jet includes a movable diaphragm positioned within a chamber. Movement of the diaphragm pulses air in and out of the chamber through an orifice. Prior art synthetic jets typically incorporate a piezoelectric diaphragm, which favors oscillation frequencies between 1 kHz and 14 kHz. These piezoelectric-based synthetic jets consequently have relatively low displacement and energy output capabilities. Piezoelectric devices are typically used in the loud speaker industry for high-frequency sounds where the energy level is relatively low. Although synthetic jets have been used in the prior art for mixing two streams of air, synthetic jets have not, to Applicants' knowledge, been incorporated on aerodynamic surfaces for providing aerodynamic control and/or reducing BVI noise.